1. Field of the Invention
This invention relates to an improvement of an AM stereophonic domodulator circuit for an amplitude/angle modulated signal which is obtained by angle-modulating a carrier wave by a stereophonic difference signal and then amplitude-modulating the angle-modulated carrier by a stereophonic sum signal.
2. Description of the Prior Art
There are two types of amplitude/angle modulation, i.e. AM/FM and AM/PM, as described in U.S. Pat. No. 3,167,614 (issued to Francis R. Holt and Jack Avins on Jan. 26, 1965). In the AM/FM type, a carrier wave frequency-modulated by a stereophonic difference signal (L-R) is amplitude modulated by a stereophonic sum signal (L+R). In the AM/PM type, a carrier wave phase modulated by a stereophonic difference signal (L-R) is amplitude modulated by a stereophonic sum signal (L+R).
When the AM/FM or AM/PM type is employed, the antenna input level/output level characteristic of the AM detected signal is different from that of the FM or PM detected signal in the stereophonic demodulator circuit, and thus the AM detected output signal (L+R) and the FM or PM detected output signal (L-R) have different amplitude levels. This involves the problem that when the left channel signal (called L signal hereinafter) and the right channel signal (called R signal hereinafter) are extracted or separated from the respective detected stereophonic signals (L+R) and (L-R) in an audio matrix circuit, the degree of separation of the L and R signals is degraded with relatively low and relatively high level antenna inputs.